Figure 2 sketches a state diagram of a LTE UE. When the device
is powered on and reachable, the possible states are RRC_CONNECTED
and RRC_IDLE. In the former state, the UE has a Radio Resource
Control (RRC) connection established with the eNodeB. In that
mode, if DRX/DTX is not used, the UE listens to every sub frame
and continuously draws a significant amount of power. With DRX/DTX
activated, the UE monitors the subframes only during the specified
timer intervals. With DTX, after the data is been transmitted the
UE turns off the TX circuit and checks for availablity of new data
to be transmitted periodically. Similarly, once DRX is activated and
if there are no packets received for a long enough duration, i.e. the
RRC_Inactivity timer expires, the UE transitions to the RRC_IDLE
state in which the UE draws comparatively little power because it
only wakes up periodically to receive control sub-frames. Inactivity
timer value is set by the network operator with a typical value of
10 seconds[9], which means that the UE typically consumes constantly
high power for 10 seconds after completing a data transmission.
This energy wasted is sometimes referred to as tail energy
and it also exists in 3G networks[2].